buildroot: run tee-supplicant as non-root

Create user and group tee/tee and set the proper permissions on /dev/tee*
and /data/tee so that tee-supplicant and the client applications may be
run as a non-root user.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>